Mysql全文搜索和group by一起很慢

Question

我有一个包含以下索引的表：

PRIMARY KEY (`msg_id`),
KEY `ix_msg_user_id` (`msg_user_id`),
FULLTEXT KEY `ix_msg_text` (`text`)

如果我只进行正常的全文搜索，则速度很快并使用全文索引：

SELECT * FROM msg WHERE MATCH (msg_text) AGAINST ("some text" IN BOOLEAN MODE);

关于该查询的解释是：

+----+-------------+--------+----------+----------------+----------------+---------+------+------+-------------+
| id | select_type | table  | type     | possible_keys  | key            | key_len | ref  | rows | Extra       |
+----+-------------+--------+----------+----------------+----------------+---------+------+------+-------------+
|  1 | SIMPLE      | tweets | fulltext | ix_msg_text    | ix_msg_text    | 0       | NULL |    1 | Using where |
+----+-------------+--------+----------+----------------+----------------+---------+------+------+-------------+

现在，如果我执行相同的查询，但是添加一个组，则需要很长时间：

SELECT max(created_at) FROM msg WHERE MATCH (msg_text) AGAINST ("some text" IN BOOLEAN MODE) group by msg_user_id;

该查询的解释是：

 +----+-------------+--------+----------+----------------+----------------+---------+------+------+----------------------------------------------+
 | id | select_type | table  | type     | possible_keys  | key            | key_len | ref  | rows | Extra                                        |
 +----+-------------+--------+----------+----------------+----------------+---------+------+------+----------------------------------------------+
 |  1 | SIMPLE      | tweets | fulltext | ix_msg_text    |   ix_msg_text | 0       | NULL |    1 | Using where; Using temporary; Using filesort |
 +----+-------------+--------+----------+----------------+----------------+---------+------+------+----------------------------------------------+

我尝试添加另一个包含text和user_id的索引，如下所示：

ALTER TABLE ADD FULLTEXT INDEX `msg_msg_user_id` (`msg_text`, `msg_user_id`);
ERROR 1283 (HY000): Column 'msg_user_id' cannot be part of FULLTEXT index

但是，它不会让我失望，因为msg_user_id是一个int并且不能成为全文索引的一部分。加快查询速度的最佳方法是什么？